How to Resize a Vector Without Initializing New Elements in C++?

In C++, vectors are dynamic arrays because they can change their size dynamically during the insertion and deletion of elements. In this article, we will learn how to resize a vector without initializing new elements in C++.

Example

Input:
Vec ={1,2,3,4,5} , capacity=5
Output:
// Resized vector 
Vec ={1,2,3,4,5}, capacity=10

Resizing a C++ Vector Without Initializing New Elements

To resize a vector without initializing new elements, we can use the std::vector::reserve function provided by the STL that changes the capacity of the vector, but not its size. So, by using std::reserve, we can avoid unnecessary reallocations by allocating enough memory upfront and avoiding the initialization of the new elements.

C++ Program to Resize a Vector Without Initializing New Elements

The below example demonstrates how we can resize a vector without initializing new elements in C++.

C++

// C++ Program to resize a vector without initializing new
// elements
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    // Intialize a vector
    vector<int> vec = { 1, 2, 3, 4, 5 };

    // Print the original vector
    cout << "Original Vector:";
    for (auto ele : vec) {
        cout << ele << " ";
    }
    cout << endl;
    // Print the original size and capacity of the vector
    cout << "Original size: " << vec.size() << endl;
    cout << "Original capacity: " << vec.capacity() << endl
         << endl;

    // Resize the vector using reserve() function
    int newSize = 10;
    vec.reserve(newSize);

    // Print the resized vector
    cout << "Resized Vector:";
    for (auto ele : vec) {
        cout << ele << " ";
    }
    cout << endl;
    // Print the size and capacity after calling reserve
    cout << "Size after reserve: " << vec.size() << endl;
    cout << "Capacity after reserve: " << vec.capacity()
         << endl;

    return 0;
}

Output

Original Vector:1 2 3 4 5 
Original size: 5
Original capacity: 5

Resized Vector:1 2 3 4 5 
Size after reserve: 5
Capacity after reserve: 10

Time Complexity: O(N) where N is the size of the resized vector.
Auxiliary Space: O(N)

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